FASE I: Conocimiento Preliminar

3.1 Introduction

Hyperbranched polymers are polydisperse materials which may have similar physical properties to dendrimers. Their properties differ from linear and crosslinked polymers. For example, they tend to have lower melt viscosities and increased solubility compared to similar molecular weight linear polymers.

T o compare the properties o f dendrimers and hyperbranched polymers, w e wished to prepare hyperbranched polymers constructed from the same branch unit as previously used in the preparation o f dendrimers. Dendrimers are prepared by a repetitive multi-step procedure which often requires chromatographic purification at each stage. Hyperbranched polymers on the other hand, may be prepared in a one-step fashion. They are thus far more amenable to large scale preparation than dendrimers and consequently more attractive for industrial applications.

At the outset o f our work, there w ere very few publications on the preparation o f hyperbranched polyesters. There were no examples o f the use o f 5-hydroxyisophthalic acid as a monomer for the synthesis o f hyperbranched polyesters but there w ere tw o examples o f hyperbranched polyesters derived from 3,5-dihydroxybenzoic acid 46.79, 80

Frichet et a l reported the one-step synthesis o f hyperbranched polymers by first converting 46 to the silyl ether-ester 7 5 using trimethylsilyl chloride (TMSC1) and triethylamine (schem e 3.1).79 The silyl ester was then converted to the acid chloride 76 using thionyl chloride.

Scheme 3.1

46 75 76

Thermal polymerization o f 76 at a variety o f temperatures gave the silyl terminated hyperbranched polyester 77 (schem e 3.2). The silyl groups w ere rem oved by stirring in pyridine-benzene to g iv e products with Mn ranging between 16000 and 55000 and polydispersities ranging from 1.9 to 3-8 (78, schem e 3.2). The highest polydispersities occurred w hen longer reaction times w ere em ployed at lower (-2 0 0 °C) temperatures. Conversely, the lowest polydispersities w ere observed at higher reaction temperatures (-260 °C ) and short reaction times. T h e molecular weights o f the polymers w ere inversely proportional to the temperature o f polymerization, thus the highest molecular weights w e re obtained at 190 °C and the lowest at 260 °C. The degree o f branching was determined by NMR and found to be between 55-60%. In a subsequent publication, Fr6chet et a l reported the improved chloride free preparation o f 76 using hexamethyldisilazane (H M D S) instead o f TMSC1.109 The absence o f

chloride im proved the stability o f 76, allowing greater reproducibility in the preparation o f hyperbranched polymers.

Scheme 3.2

77

J

78

Turner et a l prepared all aromatic hyperbranched polyesters by modifying

with acetic anhydride. Polymerization o f 79 in the melt (schem e 3.3) ga ve a brittle material, which upon purification, gave the polymer as a w h ite solid (M n 33000, PD i 3-9). The acetate groups w ere rem oved by partial hydrolysis using hydrochloric acid (M n 29000, PDi 3-3). It was estimated by NMR that the hydrolysis rem oved approximately 85-89% o f the acetate groups. M ore vigorous conditions w ere not em ployed as Turner feared this w ould lead to degradation o f the polym er backbone.

S chem e 3*3

R - C H jC O or H

3.2 Poly(5-hydroxyisophthalic acid)

W e decided to investigate the synthesis o f hyperbranched polymers using 5-hydroxyisophthalic acid

41

46.80 The hydroxyl functional groups w ere converted to acetate esters 79

to synthesize dendrimers (DCG-DPTS). This method would, if successful, have the advantage o f being carried out at room temperature and using a commercially available monomer instead o f a silyl or acetyl derivative.

Esterification o f 41 in dichloromethane or acetone resulted in the formation o f a solid which was insoluble in acetone, methanol and THF. The insolubility o f this material in THF meant that no GPC data w as obtained. Attempted dissolution in THF overnight also failed. T h e insolubility may have been due to anhydride formation caused by the dehydrating agent DCC. Turner remarked that anhydride formation in his hyperbranched polyesters resulted in insoluble crosslinked networks.81

It was at this time that w e were also experiencing solubility problems with our 5-hydroxyisophthalic acid dendrimers and it was thus decided to investigate a system which was known to be soluble.

3.3 Poly(3,5-dihydroxybenzoic acid)

3.3.1 Method 1

W e decided to investigate the synthesis o f hyperbranched polymers based upon 46, using the DCC-DPTS esterification agents. These were known to be soluble in solvents such as THF and thus were less likely to present problems o f solubility.79,80

GPC analysis o f the crude product o f the polymerization o f 4 6 (in the presence o f DCC and D PI’S) indicated the formation o f poly(3,5- dihydroxybenzoic acid) 80. Samples w ere taken 10 mins, 30 mins, 1 hr, 2 hrs, 4 hrs and 24 hrs after the reaction had started and analyzed by GPC. After 30 mins, no further change was observed by GPC, indicating reaction completion. However, the thick white dicyclohexylurea (DCU) precipitate normally seen for DCC-DPTS reactions, was not observed until 1 hr after the reaction had started. Filtration o f the crude mixture rem oved the DCU by-product and precipitation by dichloromethane gave 80 as an o ff white solid (M n = 1800, PDi = 1.09). Analysis by 'H and 13C NMR indicated the presence o f a high proportion o f DCC. All attempts to rem ove traces o f DCC by precipitation from a variety o f solvents (dichloromethane, ethyl acetate and w ater) failed. Heating 80 also failed to sublime the excess DCC.